Unlikely salt damage - most likely the concrete was an inferior mix or was badly laid or both and there has been some water ingress with resulting frost damage.
It can be successfully patched but the trouble with patching concrete is the repair has to have a feather edge and concrete doesn't feather edge well and a cement feather edge doesn't age well at all.
To overcome this fairly common problem there are a variety of dry and ready mixed compounds designed for concrete repair that are variously reinforced with resins and/or fibres. They are convenient but quite pricey.
I prefer to use the old fashioned method and overcome the problem of the feather edge by chiseling out more of the original concrete to make a stepped edge at least an inch deep and to make the cavity a fairly even depth of at least an inch deep leaving only good secure concrete behind.
Once the cavity is prepared and free of dust, using a good quality pva adhesive, brush it well into the repair area including the stepped edge and mop out any surplus. Mix enough concrete for the patch using a suitably sized aggregate of 1/2 - 3/4 inch and add a little pva to the mix according to the instructions and fill the cavity, tamp well down, screed off level with the surrounding area and trowel the surface to a similar finish to the original.
Further brushing or trowelling might be needed later.
Hello. The typical volume-ratio for a self-mix concrete in this application would be one part Portland cement to three parts sand. For a stronger pad (capable of higher stress loads) it would be as low as two parts sand, but this is not necessary for a standard patio pour. The gravel should be three parts, more will make it tougher to achieve a smooth finish. Re-cap: Portland cement - 1 part, sand - 2 or 3 parts, gravel - 3 parts. Hope this helped.
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